In the fields of digital audio and telecommunications, among others, it has become increasingly desirable, in converting from digital to analog signals, to produce analog signals of low distortion and noise. This desire has led to the common use of DACs which perform "digital oversampling". "Digital oversampling" is a technique by which there is provided from a digital input signal of a certain resolution, provided at a first rate, a second digital signal of like resolution but at a second rate which is significantly greater than the first rate. Typically, a digital oversampling element receives a digital input having an input sampling rate and outputs digital samples at a greater rate than the input sampling rate. "Analog oversampling", by contrast, is a technique in which an analog signal is sampled at greater than the Nyquist rate. Analog oversampling is commonly performed in analog-to-digital converters.
Digital oversampling DACs typically utilize sigma-delta architecture. DACs utilizing sigma-delta architecture commonly include an interpolator which receives a digital input signal and increases the sampling rate (typically 64 times the input frequency) of the digital input signal, producing a higher frequency output signal. A sigma-delta modulator receives this higher frequency output signal from the interpolator and converts the received signal to a lower accuracy (typically one bit), high frequency signal. Additionally, the sigma-delta modulator performs "noise shaping" on the signal input thereto. "Noise shaping" is a technique by which the noise spectrum of a signal is manipulated and most of the noise power is moved to a frequency band substantially outside of the signal bandwidth. The sigma-delta modulator essentially acts as a high pass filter to quantization noise and a low pass filter to the input signal. This process, referred to as "sigma-delta modulation", is purely a digital process for a DAC.
The one-bit data stream output by the modulator is converted to an analog signal and fed to a low pass filter which acts to filter out some of the high frequency quantization noise. The resulting output signal is a low distortion signal with a very low in-band (i.e., signal bandwidth) noise component, desirable for most applications.
With prior art sigma-delta DAC systems, if a user intends to reduce the input sampling frequency by a certain factor to suit a particular application (i.e., from a digital audio application to a telecommunications application), the user must change the system clock frequency by the same factor. In other words, the operating frequency of the sigma-delta modulator has to be reduced to handle the reduction in the input signal frequency. The result of the reduction in the operating frequency of the modulator is that the shaped quantization noise resides in a lower frequency band.
In a typical digital audio application, such as a compact disc player, the input signal has a relatively high input sampling frequency (for example, 44 kHz) and, thus, the quantization noise is shaped within a frequency band substantially outside of the audible frequency range (assuming an interpolation ratio of 64). When the input sampling frequency is reduced (to a rate such as 5.5 kHz), to suit a telecommunications application, a corresponding reduction in the operating frequency of the modulator results in the quantization being shaped substantially within the audible frequency range. This result presents a clear problem as it is undesirable to have a high level of noise within the audible range for a telecommunications application.
One potential solution to this problem, as used in prior art approaches, is to use a complex filter (i.e., high order filter) to reduce the quantization noise to either an inaudible level or a level suitable for a particular application. A drawback to this solution, however, is that such a filter is much more costly to implement than a more simple filter.
Another potential solution to this problem, as used in prior art approaches, is to increase the oversampling ratio of the system for all input sampling rates. A drawback to this solution, however, is the extremely high clock rates of certain components necessitated by high input sampling rates.
Accordingly, a general purpose of the present invention is to provide an improved oversampled sigma-delta DAC system having a modulator which maintains operation at a relatively high clock frequency despite a substantial reduction in the input sampling frequency.
Another purpose of the present invention is to provide such a system in which the modulator shapes quantization noise in a substantially inaudible frequency range despite a reduction in the input sampling frequency.